The heart of a computer's long term memory is an assembly that is referred to as a magnetic hard disk drive. The magnetic hard disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm adjacent to a surface of the rotating magnetic disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly located on a slider that has an air bearing surface (ABS). The suspension arm biases the slider toward the surface of the disk, and when the disk rotates, air adjacent to the disk moves along with the surface of the disk. The slider flies over the surface of the disk on a cushion of this moving air. When the slider rides on the air bearing, the write and read heads are employed for writing magnetic transitions to and reading magnetic transitions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head has traditionally included a coil that passes through a magnetic yoke that includes a write pole and a return pole. Current conducted to the coil layer induces a magnetic flux in the pole pieces which causes a write field to emit from the write pole for the purpose of writing a magnetic transition in tracks on the moving media, such as in circular tracks on the rotating disk.
Traditionally a sensor such as a GMR or TMR sensor has been employed for sensing magnetic fields from the rotating magnetic disk. Such sensors use a spin valve magnetic design, including a nonmagnetic conductive spacer layer, or nonmagnetic insulating barrier layer, sandwiched between first and second ferromagnetic layers, referred to as a pinned or reference layer and a free layer. First and second leads are connected to the sensor for conducting a sense current there-through. The magnetization of the pinned layer is pinned perpendicular to the air bearing surface (ABS) and the magnetic moment of the free layer is located parallel to the ABS, but free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.
When the magnetizations of the pinned and free layers are parallel with respect to one another, conduction or tunneling of electrons through the stack of layers is maximized and when the magnetizations of the pinned and free layer are antiparallel, overall conductivity is reduced. Changes in conduction or tunneling alter the resistance of the spin valve sensor substantially in proportion to cos θ, where θ is the angle between the magnetizations of the pinned and free layers. When reading stored information the resistance of the sensor changes approximately proportionally to the magnitude of the magnetic fields from the rotating disk. When a sense current flows through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals.
In order to increase data density, manufacturers always strive to decrease the size of magnetoresistive sensors. For example, decreasing the track width of the sensor to fit more data tracks on the disk and decreasing the gap thickness of the sensor to increase linear data density. However, as spin valve sensors become ever smaller they reach a point where sensor instability and noise make the sensors impractical to achieve sufficiently high signal to noise over the required bandwidth for recording. For example, magnetic noise, resulting from the fluctuations of the ferromagnetic layers caused by temperature, can decrease the signal to noise ratio of a very small sensor to the point that such a sensor cannot effectively be used to read a signal with sufficient certainty. In magnetic tunnel junction sensors, an additional noise resulting from shot noise further increases the noise, thereby decreasing the overall signal to noise and making MTJ sensors unsuitable for ultra high density recording. Therefore, there is a continuing need for a sensor design that can be made very small for reading at very high data densities.